STUDIES
OF POI.ISHED
SURFACES
OF PYRITE,
AND SOMEIMPTICATIONS
R. L. STANTON1
Queen's (Jn'fuers,ity,K,ingston,Ontario, Canqda
Pent 1. Tnp Orrrcar- AsrsornoprsM oF Pvnnn2
AssrRAcr
In studying a large number of carefully polished surfaces of pyrite it has been found
that this mineral is, in tie general case, optically anisotropic. Interference colours are
blue-green to orange-red, and bear a constant relationship to crystal morphoJogy. On
cube faces maximum green or red is developed when cube edgesare inclined at approximately 45o to the planes of polarisation of the nicol prisms, Pentagonal dodecahedral
faces show similar interference colours, as do all other planes except the octahedral,
which are invariably isotropic.
Heat treatment experiments show that anisotropism is not irreversibly affected by
temperatures up to 570' C. It apparently bears no relation to temperature of depoiiti6n,
nor is it affected by compositional variations. It may, however, be obliterated during
polishing if harsh methods are used, and in fact pyrite may be rendered apparently
isotropic, or anisotropic, at will, simply by varying the polishing method.
The suggestionis made that the double refraction is an inherent characteristic of the
pyritgcrystal, and tlat it is due to the low structural, as distinct from dimensibnal,
symmetry of the mineral.. It is thought that the optical properties are essentially unrelated to the thermal and electrical features investigated by F. G, Smith and found
by him to depend on lineage structures,
fntrod,uct'ion
The widespreadability of pyrite to causedouble refraction of incident
light was first recognized by the writer in 1955, when, in polishing a
variety of Australian pyritic ores using a new polishing method (see
Patt 2 of this paper), it was noted that almost all material examinedrvas
distinctly air'isotropic.
Although.there are a number of referencesto this property in the
literature, it had clearly been regardedas rather dnusual and anomalous,
dartdthe sudden.generalappearanceof the feature in the Australian ores
se'erneda little surprisin& These initial observations have been followed
up udth a detailed exainination of a large amount of material and it hbs
been found that pyrite is almost invariably doubly refracting, whatever
rPostdoctorate fellow, National Research Council. of Canada.
'Part twb follows on page 103.
s/
88
THE CANADIAN MINERALoGIsT
its form or environment of formation, and that the feature is stable
over the temperature range of stability of the mineral.
Pratious Work
Bannister (1932) in discussingthe distinction of pyrite from marcasite
in nodular growths stated that 'A closer examination of two sections of
pyrite nodules. . . revealeda very weak anisotropy.Thesesectionspossessed
a higher polish than the others and neighbouring blades showed very
faint pink and blue polarisation colours . . .' He suggestedthat this was
a strain effect. Stillwell (1934) noted occasional faintly anisotropic
pyrite in zinc-leadore from Rosebery,Tasmania, and following Banrrister
consideredthe feature to be anomalousand due to strain. He suggested
that it was accentuated by the subjection of the pyrite to the heat and
pressureof a bakelite press. Smith (1940, L942), in observing variation
in the physical properties of pyrite, noted that it was occasionally
distinctly anisotropic, but he considered the feature to be generally
nullified by heating. Schneiderhdhn& Ramdohr (1g3l) make the observation that pyrite may be anisotropic as a result of arsenic impurity
and Uytenbogaart (1951and personal communication) notes that pyrite
is normally isotropic, but is occasionallyslightly anisotropic. He suggesrs,
from observationsby himself and other workers on arsenical pyritic ores
in scandinavia that the anisotropism is probably due to internal tensions
causedby arsenic or some FeS surplus. Recently McAndrew & Edwards
(1954) have noted very weak anisotropism in pyrite from Rum Jungle,
Australia, which they thought might be due to a high nickel content
(suggestedby the presenceof associatednickel minerals). Their r-ray
powder photographs showed, however, that the material was close to
"pure" pyrite and contained less than 0.5/s nickel.
Obseraotians on P yr,i,te
Altogether some350 polished surfacesof pyrite, in pyritic ores, pyritebearing rocks and as crystals have been examined.That a large number
should be considered was obviously necessaryfor statistical reasons,
and an effort has also been made to take material of as many difierent
ages and environments of formation and from as widely separated
localities as pssible. Among the ores is material from Archaeozoic to
late Mesozoicage,from orebodiesof acceptedhypothermal to epithermal
type, from Australia, the Southwest Pacific Islands and from North
America. Pyrite of sedimentary and of orthomagmatic origin has also
been examined. All specimenswere either not mounted at all, or u/ere
mounted in a cold-setting plastic at room temperature, and polishing
was carried out with diamond abrasive on stationary or slowly moving
hand pads, as described in Part 2 of this paper. There was thus no
OPTICAL
ANISOTROPISM
89
OF PYRITE
Tesr,B 1. Prnns rx Pvnrrrc OnBs
Type of Ore
Number of
Speclmens
Enmlned
Localttv
Htshly folded
Proterozoic
black shales
Acc€pted
Oricin
Hydrothermal
replac'ement
Age
Houghtonlan
1. lead-zlncopDer
Mt. Isa,
Queneland
2, lead-zlnc
Brokm Hill,
N.S.W.
2
lltghlyfolded
Archaeozolc
pelltlc rcks
3. copper
Mt. Lvell,
Tasnuia
6
Tufis md
conglomerate
,,
Lower
Carbonlferous
4. coppergold
Cobar,
N.S.W.
I
Folded StlurlanDevonlan shales,
sandstone, tufis
,,
Mtddle
Devonian
6. lad-zinccoDtg
Rmbery,
Tasmmla
12
Lower Palaeozolc
Shalc and pyroclatic rcks
,,
Lower
Carbonlferoris?
0.
Captaln's
,,
15
Host Rocks (premetamorphic
lithology)
Fl,at,
30
7,
,,
Peelwood,
N.S.W.
18
8.
,,
Wleeman's Crck,
N.S.W.
3B
Bumga,
N.S.lil.
2a
9. copD€r
UpperOrdovlclan
-Lower Siluriu
black shales md
tufis
N.S.W.
Lwer
Palaeozoic
Hydrothermal
Upper Ordovlcian-Lower
Sllurlm black
shal6 and tufig
Lows
Palamzoic
10. iopper-gold
Mt, MorgEn
Quensland
I
Devonlan (?)
lavas alrd €edl!nmta
Hydrottrermal
replacement
Late
Permlar
11. complexpolymetalllcveln
deposlt
Yeruderle,
N.S.W.
I
UpperDevodan
tavas, tufisand
shales
Hydrothermal
cavity
Glllng
Uppq (?)
Cabonlferoris
12. copp€r
Ballade Mlne,
Diahot,t New
Caledonla
10
PalAeozlc pelitic
sedlments
Hydrothemd
replacement
Palaeorctc (?)
18. gold
Fm Hlll'
Diahot, Nw
Caledonta
15
14. copper-zinclad
Mlnc,
Nmou
Diehot, New
Caledonla
l0
15. copxr-gold
Honfertr Mlne,
Plalne des Gaiacs,
New Caledonla
5
Palaeozolc tuffs
16. copper
SanJorgp leland
Brltlsh Solomon
Islards Protectorate
4
Mesozoic (?)
shales md laru
,,
Upper Mesozolc
(?)
90
THE
CANADIAN
MINERALOGIST
'TaBLE
I cant'il
Type of Ore
Number of
Specimens
Emlned
Localiry
Host Rocks (premetamorphic
lltholoey)
Amulet, euebe
18. copper-zinc
East Waite,
Ouebec
t9.
Normetal,
20. copg
Kewatin-type
lavro
Accepted
Ilydrothermal
replacement
Ace
Late
Precambrian(?)
Ouebec
Horne Mine,
Ouebec
21. gold
Hollinger,
Ontrio
Keemtin
volcanic reks and
Algoman
22. cop6t-z1ncsol'd
Flin Flon,
Mmitoba
Andesitic lavas
md pyroclasts,
quartz porphyry
-Keemtla
Pre-Cambda
(?)
23. gold
Pamour Mine,
Ontario
Temlskamlng
greywackes and
other sedlments
Algoma
24. nickel+op1nr
Levack Mine,
Ontuio
Sudbury
.Sliatlons
Pre-Cambrian
25, gold
Hydrothemal
replaement
or
igneous
segregation
Ken Addison,
Ontarlo
Temlskaming
(?) edlments
Hydrotlsmal
Algoman
20, gold
Tck-Hughes,
t7, sold
Macassa. Ont.
28. lead-zlac
Sulllyan, B.C;
Late PrF
Cambrian
sediments
Late PreCambriau
20. lead-ziaccopper
Bathurst, N.B.
Lows Palaeozolc
pelitlc md pyroclastic rcks
Palaeozoic
80. gold
Norseman,W,A.
Baslclavas,
pyfoclastics md
interelated
lron fomation
and shale
*All materlal
Ont.
from New Caledonla klndly
2
contributed
Ilydrothermal
pre-Cmbriu
by the Bureau Min.ler de la France d,outre
Mer
elevation of temperature and an absoruteminimum of distortion
during
polishing.
The range of material, excluding single crystals, is shown in
Tables 1
and 2' In all ores alm_oslail pyrite cubes, particles and aggregates
were
doubly refracting, and this was invariably the case,with tliJsJimentary
and magmatic material. corours shown varied between blue-green
and
orange-rd-the colours rirentioned by both Bannister and s;ith.
pyrite particles and crystals of the ores and other rocks
-These
courd,
of course, only be seen in random section. However throughout
the
material the range of colours shown was quite constant,.sugge-sting
that
OPTICAL
'fanr-u
No.
Nimber of
Specimos
Emmined
Locallty
31. Portland,
N.S.W.
32. Hohoro,
Guinea
88. Pt. Philip
Vlctoria
2
New
3
91
OF PYRITE
AND ORTEoMAGMATTc FrrRrrE
Nature of
Material
Cube
Host Rocks
Sedimentary
Devonlan
Carbonaceous
estuarlne
sedlments
,,
Mlocene
Flne marine
eiltstone
,,
Age
Ortein
1
Radtatt$
(collofom)
concretlou
Calcereous
marlne
shales
,,
Tertlary
84. Sydney,
N.S.W.
2
Cubs
Iacustrine
slltstone
,,
Lower
Trimic
35. Lardeau Map
Area. S.E.
Brtttsh
Columbia
1
Slieltfy
deformed
Marlne calcareous
argilUte
,,
Upper (?)
Ploterozoic
36. Broken H111,
N.S.W.
2
Cubes
Metamorphosed
calmus
pelltlc rmks
,,
Prcterozoic
87. Kakabeka Fa[s,
Thunder Bay,
Ontrio
1
Radlattnc
concretiomry
Animiki
shales
,,
Pre-Cambria
38. Bumbo Ouarry,
' Klania, N.S,W.
1
Isolated cub€
Monchlquite
dyke
ftbomagmatic
Terttarv
39. Bathurst,
N.S,W.
3
Cubes and
inerular
gnrns
Andesltebasalt lava
hhomagmatic
Upper
Ordovician
rl0. Blayney,
N.S.W.
2
lregular
gratns
Monzonite
intruded
andesite
MaFnatic and
contact
metamorPhlc
Lower
Cabonlferous
(?)
41. Major's
N.S.W.
42. Prupect,
N.s.W.
Bay,
ANISOTROPISM
2. SEorwnrranv
Crek
2
,,
4
Cubes and
ircgular
grarns
Aplite
black
md
dyke
Alkallne
-{abbro
lntruglon
(laccolith
doledte
minor
(?)
Orthomagmatic
',
T€rtiary
?)
the double re{raction was produced by an ordered and consistent feature,
very possibly bearing a constant relationship to the crystal structure'
Little or no sign of undulose extinction was observed, indicating that the
double refraction was not simply a local and variable strain effect.
In order to see whether the anisotropism did in fact bear a consistent
relationship to crystal faces a number of well-formed single crystals
v/ere then examined; the nature of these and the localities {rom which
they have been taken is shown in Table 3. The following observationso
given in outline in the table, were made:
(1) Double refraction is invariably distinct, and possibly shows its
92
THE CANADIAN MINERALoGIST
Tesr-s 3. Pynns ExaurNso rN Snrcr,e Cnysrer-s
Localtty
WO-1746
Fomsdeveloped
Home Lllne, Ouebec
Propertie
between cossed nicols
Anlsotroplc,
colours blue-green,
when cube edges in 460 posltlon.
omnge-red
Helen Miae, Ontario
PortJand, N.S.W.
Captain's
N.S.W.
Flat Mine,
North Wlsemm's Mine,
Batburst, N.S.\ry.
D-31570
Mclntvre,
ontarlo
Mclntlre,
Ontario
PrcaFt,
cubemdoctahedron
a1 above,
lube
tsoroplc
Octahedron
Isotropic
Cubeand.pentagonal
dodeehedron
Cube as above,
refncting
Pentagonal
dodecahedran
All faccs doubly refracting
ctahedral
facee completely
N.S.W.
Belllnga, N.S.W.
Valla Mine, Uruuga,
N.S.W.
D-4f59
Milhau,
D€743f
TrevemaMine,Nundle,
N.S.W.
D-3t1461
Lobb'e Hole,
Yarrangobilly,N.S.W.
D-34462
More'e
Taammla
Averron,
Fme
pyrttohedn
also
doubly
Mine, Zehm,
D€4463
Mt. Galena, Enmavllle,
N.S.W,
D-2O102
FroLJln,
New Jereey
WO-1786 Butte, Montm
*Prefx WO denotes coU.ectlon of Department
of Geotogy, Uriversity
of Western Ontario; D the collec_
tion of tle Australian Museum, Syduey; No. 607 and 281 are from F. G. Smith's material at Mills
HaIl.
Quen's Unlversity.
greatest intensity, on polished cube faces and surfaces sawn parallel to
these. with nicols nearly crossed (or, when double refraction is particularly pronounced, with the nicols completely crossed) the colours are
blue-green and orange-red, the maximum or purest red or green being
developed when the cube edges are at approximately 4bo to the vibration
directions of the nicol prisms. These colours, and the directional feature.
have been found in every case for sections parallel to the cube face.
(2) sections parallel to the octahedral faces are isotropic. Again both
lightly polished natural faces and faces produced by grinding or sawing
OPTICAI,
ANISOTROPISM
OF PYRITE
93
parallel to Lll urere examined. Where complete octahedrons were available, four non-opposing faces were polished to see whether some were
isotropic, others anisotropic. In most cases all appeared completely
isotropic. In one or two individual faces extinction was not complete, but
double refraction was so slight it seemed most likely to be due to the
production, during polishing, of a plane just off the 1L1, although a
great deal of care had been taken to keep the polished planes parallel
to the original faces.
(3) Pentagonal dodecahedral (pyritohedral) faces and sections parallel
to tJrem always showed distinct double refraction, with interference
colours essentially similar to those shown by planes parallel to cube
faces.
(a) No further different forms were available to the writer for examination, but random and oriented cuts were made in several groups of welldeveloped intergrowths, and simultaneous observations on the differently
oriented crystals made. In every case the individual crystals showed up
by contrasting colours between crossed nicols.
These observations are for the most part quite in agreement with
those of Bannister and Smith. The former had noted faint pink and blue
polarisation colours, and the latter had, in a cubic crystal from Leadville,
Colorado, noted the features of cube and octahedral planes outlined in
(1) and (2) above. These features are now, however, recorded as general.
Stability of the An'isotrop'ism
(l) The effect of heat. The only person to have considered the possible
instability of the feature is Smith (L940, 1942) who in his very careful
and detailed investigation of the various physical properties of pyrite
came to the conclusion that double refraction is shown by pyrite only
until it has been subjected to temperatures in the vicinity of 135"C.
Heat treatment above this point, he states, nullifies the feature' the
transition being irreversible over short periods.
'That heat treatment should have this effect was suggested to Smith
by the changed appearance of pyrite after mounting in bakelite, and
repolishing. He noted (1940) that some fragments polished by hand on
cloth laps showed distinct anisotropism, but that these sections, after
mounting in bakelite and mechanical polishing, were all isotropic. The
maximum temperature of the bakelite moulding die was about 135"C
and therefore, according to Smith, heating to this point was sufficient to
destroy anisotropism. Later, in attempting to determine the exact
temperature at which the change took place, Smith (L942) heat-treated
a doubly refracting cube in an oil bath to temperatures in the l-00"-214"C
range, but with no apparent effect. The possibility that larger crystal
94
THE CANADIAN MINERALoGIsT
size might have had a dampening effect on the change was then tested
(1942, p. 12) and the conclusion reached that ,the smaller the size of the
crystal fragment, the more readily the transition takes place'-an
erroneous conclusion in which, as will be shown later, polishing deforma:
tion was interpreted as a heat efiect. strangely smith then cast aside
his evidence of persistence of the feature above l3boc and came to the
conclusion that a temperature approximating to this was the critical
one.
To test Smith's assertion the writer has carried out the series of
*r""
o.*nt"t"
*orr
"*r" "*o.,r*"
No.
3l
Locality
Heating Tlme Marlmum
(r@m temp. Temperature
(2 hours)
24" C)
Nature of Material
Portland. N.S,W.
Isolated cub€
Cmllng
Time
(appror.)
4.76 hrs,
58o C.
4.26 hrs.
1360 C.
f .6 hF,
3l
4.76 hn.
2C[)oC.
2.0 hrs.
31
4.76
8260 C.
3.0 hE.
8t
4.0 hrs.
4160 C.
3.0 hre.
3l
4.O hre,
660" C.
3.0 hB.
hlF,
I
hr.
Mt. Morgan,
Oueensland
Irregular large gratn
from copper-gold ore
4.0 brs.
4 1 5 0C .
3.0 hn.
DA4462
Munroe's Mine,
Zehm, Tas.
Well-formed
pyritohedron
4.0 hrs,
6700C.
3,0 hrs.
48
Mt. Is,
Well:formed cUbes wlth
galena presure shadows
in duk shale
4.O hrs.
672oC.
3.0 hrs.
Wiremm's Creek,
Pyfitic
2.0 hn.
5000 C.
2.0 hE.
Captaln's Flat, N.S.W.
Pyritlc Cu-Zn-pb ore
1.25 hre.
6160 C.
2.0 hrs.
Hercules Mine.
Rmbery,
Tmmania
Pydtlc
2.5 hrs.
6000C.
2.0 hrs,
Franklin, New Jerrey
Pyritohedron from
Australim Museum
Collation
1.5
hrs.
6000 C.
3.0 hre.
Valki Mine
Octohedron
1.6 hre.
500" C.
3.0 hre.
1.5 hn.
5050 C.
8.0 hn.
60
Quensland
Cu-Zn-Pb
ore
copper ore
64
Yemnderie,
@
Cobr, N.S.W.
Pyrittc copper ore in
blmk shale
1.0 hrs.
600"C.
2.0 hrs.
56
Climq,
Pyritohedrons
rhodffiNlte
6.6
5300 C.
3.0 hrs.
N.S.W.
Colorado
Come pyritic
lead ore
sllver-
in
hre.
OPTICAL
ANISOTROPISM
OF PYRITE
95
heating experiments summarized in Table 4. unmounted material,
previously determined as anisotropic, was placed in a glass container
through which a stream of nitrogen could be passedcontinuously. The
whole was placed in a furnace in which temperatures could be controlled
to about plus or minus 5oC.
As shown in Table 4, the pyrite was raised to a variety of temperarures,
and both heating and cooling times were varied. In some cases the
material was quite coarse (single crystals up to 2.0 cm. across) and in
others fine. where temperatures above about 500oc were attained it
'was impossible to prevent some slight oxide tarnishing,
and specimens
D34452and 56 broke into several fragments from which, however, some
large enough for repolishing could always be selected. In no case was
any of the material mounted at any time.
When lightly "touched up" on a stationary diamond pad after heating,
all pyrite was still anisotropic-to a degree which could not be distin.
guished from that observed prior to heating. It was thus clear that for
somecasesat least heat treatment had no major irreversible efiect on the
crystal structure.
Tear.p 5. Prnrrr
Locality
wo-1746
wo-17&l
Hperoo rx Vecuo ro 5@o C.
Hone Mine, Nomda,
Quebec
Grqup of lntergrown cubes
Flln Flon Mlne, Manitoba
Masslve, very fne gralned
pyrlte
Butte, Nevada
Intergrowa well crystalllsd
pyrltohedrots
Helen Mlne, Ontarlo
Intergrown
Thundu
No loallty
Bay Dlst., Ontario
ftstn
Type of Materlal
cubes
Hy&othermal
Altered
Hydrothermal
Sphertcal concretlon
Sedlmentary-black
envlronment
Two l,arge cubes, lntergrom
Unknom
I
shale
To test the idea further, but in a more carefully controlled way, six
North American pyrites, five from well-known localities, one a well
formed intergrowth of two cubesfrom an unknown locality (seeTable 5),
were polished unmounted and found to be doubly refracting. X-ray
powder photographs all showeda normal pyrite pattern. Each specimen
was then separately sealed in an evacuated pyrex tube, placed in a
furnace and raised from room temperature to 500oC over four hours.
This temperature was held f.or 2O hou-rs,and then cooling back to roorn
temperature allowed over about 3 hours. Ileat-treatment again pro-
THE
96
MINERALOGIST
CANADIAN
duced no detectable changein the double refraction, and further powder
photographs of each indicated no change in crystal structure'
Examination of "High" and, "Love" TemperaturePyrite
As Smith had consideredthat the optical properties of pyrite might
serye as a further geothermometer (1942, p. 17) it was suggestedto the
writer by ProfessorJ. E. Hawley that some pyrite from the Mclntyre
Mine, Porcupine District, Ontario, the temperature of deposition of
which had been determined by Smith on the basis of his electrical
conductivity methods,might be examined.Twenty-four specimens,listed
TeaI-B 6. Pynrtp FoR WETCE TnUpBn-A,tUnB or DrpOSlrrON
nv F. G. Sur:ru. h.roox NuarsnRs AND ALL orUsn Dere
Sample
Number
49f
317
468
462
r64
1()8
Mitre
Level
Vein
Number
Temperature
of
dePositlon u
detemlned
bY Smlth
Desclptlonofmaterial
6226
600'lnside PorPhYry
10
0l0o C.
Irregular cystals in quartz,
hotite md chalcopyrite
6226
46{f lmlde porphyry
10
6760 C.
Irregular
masses in porphyry
6200 C.
Irregular
aeas in quartz vein
P!m-
2126
360'lnsldeporptryry
4926
3@'lmlde
porphyry
13
6000 C.
Masslve veinlet ln quartz-feldspar
dn
n76
270'inslde porphyry
10
476o C.
Dissemlnated
6876
220'lnsldePorPhYry
t26o C.
Inegular
10
5100 C.
Cubes in grey schist
4600 C.
S€ttered
4f 0o C.
Small obes in quartz
porphyry
obes
ln grey schlst
ln quartz
vein
2a76
70'lrelde
49
30'lnside porphyry
87ffi
Contact
4626
6, outslde porpbyry
6100 C.
Thin velnlet in blac-k slate
4626
l0'outslde
porphyry
4300 C.
Irregular
groups tn black slate
1600
610'outsldeporphyry
0300 C.
Irregular
particles ln quartz
60'outside PorPhYry
1000 C.
cubes along
Striated
quartz veiB
1260 C.
Lage
1250 C.
Mmive
320
L760
26r
m
00'outside
70'outslde
480
t&
Dlstuce from
porphyry-sediment
boundary
EAS BEEN DBTERMINED
AcConOrNC tO sUrtH
6rm
lo
porPhyry
13
DorPhYry
cube ln grey schist
cublc crlntals
border
in quartz
velnlet ln gremtoae
l(F'outsideporphyry
176'C.
Minute cubes in quartz strings
1600 C.
Small cubesin
f26" C.
Dissemlmted
1260 C,
Small cube in quutz
396
AZW
110'outsldePolPhYry
484
3sm
l3d outside porphyrv
178
L626
220'outsldepof9hyrY
13
grey schbt
cubo ln grenstone
of
OPTICAL
ANISOTROPISM
OF PYRITE
97
in Table 6, were taken. Number, position, temperature and occurrenie of
each specimenare given as by smith. Each specimenwas mounted in a
cold-setting plastic at room temperature and pressure, and each was
polished using the procedureoutlined in Part 2, againensuringthat there
was no elevation of temperature and a minimum of distortion during
both mounting and polishing..In every case,for both ,,high" and ,,low"
temperature material, the pyrite was distinctly anisotropic.
From this experimental and microscopicevidenceit seemsreasonable
to conclude that the optical aaisotropism of pyrite is not generally
irreversibly affected by heating-to temperaturesup to bbG-bz0"c rangl
-and that previously there has treensomefactor other than heat
causing
loss of observableanisotropism.
(2) The efect of polishing and,swface d,eformation It is of some interest
to consider smith's statement (1940, p. 91) 'It was found by accident
that heat treatment of an anisotropic specimenrendered it isotropic. A
fragment of the same crystal as 22 (a large cube from Brosso, ltaly,
R.L.s.) was gound flat and polishedby hand on cloth laps . . . . It showed
distinct anisotropisrr, the colours with the nicols crossed being light
blue-green to light orange-red.Seven sectionswere mounted in bakelite
for rnechanicalpolishing, three sections parallel to the three cube-faces,
and four sections perpendicular to the body-diagonals of the cube. AII
were isotropic. The maximum temperature of the bakelite moulding dib
was 135"c and cooling to about 100oc took place in b minutes,' and his
conclusion that heating during mounting causedloss of anisotropism.
on analysis his procedure and result present two possibilities; that
disappearanceof optical anisotropism has been caused'by:
(o) the increasein temperature suffered by the:pyrite in the bakelite
press, or
(6) some deformatory action in the second polishing processavoided
in the first.
As the present writer's.first observations of general and pronounced
anisotropism were made only after a change in polishing techniq.ue,it
naturally occurred to him that the previous isotropic appearance of
pyrite might have been due to surface deformation produced by other
polishing methods. In other words, smith had been presentedwith two
possibilities-deformation during mounting or during polishing-and.
had assumedthe first when the second may have been the case. This
idea was then tested.
(a) Repolishing of material already polished on metal laps-pyritebearing material earlier polished by the writer on the lead laps of 'the
Mineragraphic Section, C.S.I.R.O., Melbourne,3 were carefully re.
lBy courtesy of Dr. A. B. Edwards.
98
TSE
CANADIAN
MINER.A'LOGIST
examined and the pyrite found to be isotropic. The polish was then
removed by cutting back gently with American Optical Co. 304| emery
on glass, and the pyrite repolished using the new method. All material
immediately appeared anisotropic. The same specimens were then
repolished on lead laps at the University of Sydney, and when reexamined were isotropic. It was thus found that the apparmt optical
properties-for thesespecimensat least- could be changedat will simply
Ly changing the polishing method. A small amount of confirmatory work
on North American material has been carried out at the University of
Western Ontario, with similar results. Material from Montgay Township,
Quebec,earlier polished on cloth laps at Queen's University and found
tL be anisotropic, then polished on lead laps at the Canadian Geological
Survey laboratories and found to be isotropic, rryasrepolished by the
writer and rendered anisotropic again. Further material (all kindly lent
to the writer by Professor Hawley of Queen's University) from TeckHughes, Kerr-Addison and Macassa Mines, previously mounted in
bakelite and polished on metal and high-speedcloth laps was found to
be just slightly doubly refracting. When repolished by the writer anisotropism becamequite prominent, and the normal coloursand relationships
displayed.
ft *6 thus clear from these experiments that it was some feature of
the polished surface, rather than variation in the body of the mineral,
that was causing differencesin optical behaviour.
(D) Testing.of surface efiects- The conclusionmost easily seized(and
in fact.the one now finally adopted) was that earlier polishing methods
'surfaces, making these.afparentl'y
had been deforming the mineral
isotropic, and that the more recent methd, in causing a m'inimum of
deformation, revealed the true anisotropism. On the other hand it has
been pointed out by Perryman & Lack (1951) that polarisation effects
in cubic metals (such as aluminum) can be produced by deeply etching a
polished surface- i.e.that surfacecontour, and hencepolishing artifacts,
rnay produce anomalousdouble refraction. Whether or not polishing is
producing such effects can be tested by depositing (from vapour in a
"16"uu. chamber) a thin film of silver over the surface in question. Such
a film should be just thick enough topreventtransmission of normally
incident light, but sufficiently thin to take up the surface contour"of
the surface on which it is deposited.Where polishing is not at fault, the
silver film should of coursebe isotropic.
Several polished surfacesof apparently anisotropic pyrite were finely
coated in this way. All were rendered isotropic, indicating that double
refraction was prduced by the mineral itself and not by surfacedefect.
(3) The effect of saTiat'ion i,n composit'i'on The now established fact
OPTICAL
ANISOTROPISM
OF PYRITE
99
that pyrite is doubly refracting is clearly difficult to reconcile, at first
sight anyway, with its accepted cubic symmerry.
The most common explanation of anisotropism in pyrite is that it is
due to lattice strain caused by the presence of impurities-notably
arsenic. Smith (1942) has suggested that variation in the properties of
pyrite may be a function of two variables-composition
and secondary
crystal structure. He has shown that pyrite generally does not conform
to the ideal composition FeSz and that variation in composition is always
in the direction of sulphur deficiency within the limits FeSz.o-FeSr.nu.
He
suggests that iron takes the place of some sulphur in these crystals, and
that this affects lattice dimensions and probably also symmetry. He
suggests too that iron replacing sulphur in sulphur-deficient pyrite may
be in regular positions, giving optically and electrically anisotropic
crystals, or it may be in irregular positions, giving an optically isotropic
variety.
careful determinations of the trace element composition of the six
North American pyrites already referred to have been made. cobalt,
nickel, molybdenum, titanium, vanadium and chromium have been
estimated by careful quantitative spectrographic methods, and arsenic,
antimony, bismuth, copper, zinc, lead and silver by qualitative spectrography, The results are shown in Table Z.
Copper, zinc and lead are more or less ignored as it is well-known
that it is almost impossible to obtain pyrite without a few inclusions of
their common sulphides. The highest cobalt and nickel figures are of
the order of. 0.006/6 and so these metals cannot be regarded as significant; those for chromium, vanadium and molybdenum are even lower.
An interesting feature of the qualitative determinations is that iri
specimens 2 and 5 no arsenic whatever could be detected, and in all the
others it was very low-a feature commented upon by smith for some
of his pyrites. It is assumed for the time beinga that the iron and sulphur
figures are in the range obtained by Smith.
The results show clearly that arsenic is not the cause of anisotropism
in at least two of the specimens, and its occurrence in the other four
is quite negligible. The other elements too are present in such minute
amounts that the possibility that they are causing significant lattice
distortion cannot be seriously entertained. It seems that the deficiency
of sulphur, as pointed out blu Smith, is the only possible significanf
feature. A point that should be recognised, however, in any consideration
of possible effects of composition variation is that the double refraction
_ 4ccurate sulphur and iron determinationsfor thesepyrites are being carried out,
but_owingto delayshad not beencompletedin time foi-inclusionin this paper.The
work is continuing,however.
TIIE cANADTAN MrNERALocIsr
100
Tenr-r 7. Spectrographic determinations of trace elements in the six North American
pyrites listed in lrUtu S. Quantitative determinations are each the average-of five
Lutns. For qualitative: vs : very strong; str - strong; m : medium; w :.w.eak; tr
trace; ftr : faint trace; nd - not det-ected. Analyst: J. G. MacDonald, Queen's
Urt*-ta".
,
=
Element
Co
C!
Nl
3170.3
3185.4
2281'1
2349'8
2860'4
O.W%
O'OlVo
O'l%
Wavelensth
3433.0
UL4.7
4264.3
Semttivtty
O.N2%
O.NI%
O.(f{lAVI O.@l6Vo O.N4%
HomeMlne
O.OS7VIsO.Nl%
(No. wo-tzao)
<O.NlVo
O.N427o<0.W47o
Flln Flon
0.009
(No. wO-617)
<0.001
<0.001
0.0036
Butte
0.006
E0.001
<0.@1
0.0039 <0.004
llelen
(No.57)
0.006
<0.001
0.ffi22
0.w42
Thunder Bay
0.006
0.0066 <0.@t
0.0043
0.0062
0.007
0.@47
Pb
Cu
(worzao)
As
As
lVIo
nd
nd
0.0060
nd
(No. sa)
No Lmltty
(No.69)
Element
0.008
Zt
<0.@2
Sn
Wavelength
2&it!.0
8247'6
3345.0
28it0.9
Semitlvlty
O.WlVo
O.ML%I
O.Ol7o
O.@L%o
0.0048
Bi
Ag
3280.0
O.@lVo
I{ome MiDe
(No. WO-1746)
tr
str.
str.
F'lln Flon
(No.wO-017)
str.
str.
str.
Butte
(wo-1786)
nd
Helen
(No.67)
tr
m
Thunder Bay
(No. 68)
atL
str.
2670.8
3447.7
o.w3%
nd
ftr,
nd
No Locallty
(No.69)
nd
nd
nd
nd
nd
no
has been found to be very uniform over the whole of the wide range of
material examined. This alone clearly throws extreme doubt on the
possibility that iron : sulphur ratios or trace element assemblages could
be significant.
Dis cuss'ion and, Concl'us'ions
It is now considered established that:
(1) Pyrite is in general optically anisotropic.
OF PYRITE
OPTICALANISOTROPISM
101
(2) Double refraction is produced by any plane other than the octahedral.
(3) Anisotropism is not irreversibly afiected by heat up to 570'C at
atmospheric pressure.
(4) Pyrite may be renderedsuperficially isotropic by surface deformation during polishing.
(5) .Anisotropism is not necessarilydue to the presenceof arsenic or
other tttrace" impurities.
The reason for the apparent incompatibility of the findings of Smith
and of the present writer concerning the behaviour of the optical anisotropism of the pyrite under heat treatment is now clear; differencesare
attributable quite simply to earlier limitations of technique. On the
other hand as it is now evident that there is no order-disordertransformation at 135oC,the suggestionthat the optical nature of the pyrite might
be used as a further geothermometermust be discounted.This is emphasized !y the writer's observations on material, all of which is doubly
refracting, that Smith himself has measuredfor conductivity and stated
to be of high temperature origin.
The continuity of the optical anisotropism fits with Smith's determinations of electrical and thermal properties. In view of his findings
that specificresistancevaried (1) with different crystallographic orientation and (2) with variation in the Fe : S ratio, and the fact that temperature-resistivity curves of crystalline solids frequently show anomaliesat
transition points (Kittel, 1953) it might have been expected that an
order-disorderchange sufficient to causea pronouncedchange in optical
properties would also produce a fairly sharp discontinuity in the conductivity curyes. Smith's text figure (1942, p. 4, fig. L) however, shows
a remarkably smooth pair of curyes with no sign of a break in the
135oCarea. In view of this, any pronounced change of lattice arrangement near this tcmperature would have seemedanomalous.
The suggestionthat the pyrite might be trigonal, although apparently
fitting the electrical and thermal properties, is not supported by the
optical data. As far as can be determined all octahedral facesare isotropic
which clearly is not compatible with the trigonal uniaxial symmetry.
The idea that regular replacement of some sulphur atoms by iron leads
to lattice distortion and the development of trigonal symmetry, which
in turn guidesthe developmentof lineagestructures and the non-isometric
symmetry of electrical and thermal properties is quite an elegant one'
but it allows non-double refraction along only one body diagonal of the
cube. If trigonal symmetry did hold the other three pairs of octohedral
faces,being essentially parallel to the optic axis, should show maximum
double refraction-which they unquestionably do not.
It seems that an explanation may lie in the low symmetry of the
102
THE CANADIAN MINERALoGIST
pyrite structure, and that the double refraction might have been predicted on theoretical grounds. It is known that the crystallogralhic
axes, although yielding four-fold d,:rnms,i,onal symmetry are, when
structure is considered, only two-fold. such a rotational symmetry
would seem to fit, and may possibly explain, the observed features of
double refraction, wherein similar colours appear twice in every 860"
rotation and in constant relationship to cube edges. By the same reasoning other planes such as 110, 210 etc. should also be expected to produce
double refraction. Octahedral planes, on the other hand, normal to
three-fold axes of symmetry would not be expected to show the double
refraction features of the other orientations.
That lineage structures could cause un'iforrn double refraction over a
whole crystal face (up to 4.0 sq. cms. in area), that they could cause
the same intensity and order of double refraction on all faces of an
individual cube, and that they could be the cause of essentially the
same intensity and order of double refraction over the whole of a large
body of samples seems unlikely. Optical properties are dictated by the
st-ructure and packing of the units of a crystal, rather than by the habit
of large scale growth of aggregates of units. A lineage structure is, comparatively speaking, a coarse feature of growth and leads to the development of fairly large-scale imperfections and gaps in the body of a crystal
as a whole. It does not seem likely that so coarse andsoaar,inDlea structure
could produce so uniform and so constant an effect on so delicate a
phenomenon as the rotation of incident light by the few upper layers of
a crystal. on the other hand density and continuity of crystal "branches"
and their degree of lateral contact would undoubtedly afiect conductivity,
and the lineage structure idea seems a good explanation of the temperature-conductivity relationships.
Thus it is considered that the optical anisotropism and the electricalthermal anisotropism of pyrite are essentially unrelated. The former
results from the basic crystal structure of the pyrite, whereas the latter
results from the grosser structures in which the'former becomes involved.
The first feature is very nearly a constant, the second highly variable.
Thus the optical properties of pyrite are ordered and constant, whereas
the electrical and thermal properties show only a rough order within an
individual crystal and an enormous variation over a large range of
material.
Acknmuled,gments
Facilities for this investigation have been provided by the Departments
of Geology of the University of Sydney, the University of Western
ontario and Queen's university, and the assistance of these institutions
is gratefully acknowledged. In particular the writer wishes to acknowledge
OPTICAL
ANISOTROPISM
OF PYRITE
103
the help of ProfessorJ. E. Hawley and Dr. L. G. Berry of Queen'sUniversity and Dr. C. G. Suffel of the University of Western Ontario, in
discussion and in their critical reading of the manuscript; and the
assistanceof Mr. R. O. Chalmersof the Australian Museum, who very
kindly lent some excellentsingle crystals.
RBrsnrNcBs
Ber.rNrsrsn, F. A. (1932): The distinction of pyrite from marcasite in nodular growths'
Mi.n. Mag23, t87.
Gnuuon, J. W. (fSm): Structures of sulphides and sulphosalts. Arn. Min. 14, 470.
state physi,cs.New York, John Wiley &
KttrEL, ise*r,Bs (1953): Introd'uctdonti sol,i.d'
Sons Inc.
McAwonow, J. & Eowenos, A. B. (1954): Bismuth and nickel minerals from Rum
Jungle, Northern Territory. C.S.,I.R.O.M'ineragraphic Imtestigatiow, Report No'
'Al.
PannyMAn, E. C. W. & Lecr, J. M. (1951): Examination of metals by polarised light.
Nature, 176, 479.
Scsr.reroBRrdnr.r,H. & Reuoosn, P. (1931): Leh.rbuchtler Erzmihroshopie, Bd' ll,
Berlin.
Surru, F. GoRDow (1940): Variation in the electrical conductivity ol pyrite. Uni'tt.
Geol,.Seri'es,M,8.
Toronto Stuil,i,es,
(1942): Variation in the properties of pyrite. Am. Min.27,I.
SraxroN, R. L. (1955): The anisotropism of pyrite. Aust. Jour, Sci' l8' 98.
StllLwELL, F. L. (1934): Observations on the-zincJead lode at Rosebery, Tasmania.
Proc. Aust. Inst. Min. & Met.94, 43.
UyreNnocAARDr, W. (1951): Tabl,esfor the m,icroscopic i,il.entifuation of ore mineral,s.
Princeton University Press.
Pent 2 - TnB Por,rsunvc Mrtsoo
AssrRAcr
It has been shown in Part 1 of this paper that pyrite is generally optically anisotropic,
but that double refraction may be modified or destroyed by polishing deformation.
In this section the nature of such deformation in metals and minerals is coirsidered,
and the disadvantages of current mineralographic techniques discussed. An efficient
method of mineral poli.tri"g, based on the use of a wax-abrasive mixture and diamond
impregnated naplesi cloths, and capable of producing a hish polish with little relief
lo* surface deformation, is then described. Several microphotographs indicating
"nd
the quality of results are shown, and some implications of the improved method
suggested.
Introd,uction
Although until now all that has been required of a polishing process
in mineralography is that it should yield a smooth highly reflecting
surface,it has been shown in Part 1 of this paper that requirementsare in
fact two-fold. A fine, highly reflecting finish is certainly-and obviously
104
THE
CANADIAN
MINERALOGIST
-required, but it is also important that the surfaceyielded should be as
representativeof the underlying mineral structure as possible.It appears
that in.work done to date observations have been made on variablyand sometimesbadly-deformed versionsof minerals, rather than on the
natural minerals themselves.
It is the purpose of this section of the paper to consider the nature of
these deformed layers, the mechanism of their formation and ways in
which their excessivedevelopment may be avoided. Reference is also
made to some preliminary results obtained in the polishing of cubic
mineralsother than pyrite, and someimplicationsof polishingin hardness
and reflectivity determinations.
Stud.iesin M etal,lograpbi,cP olish.i.ng
In considering the effects of polishing on minerals, studies of metal
surfaces,although not strictly analogous,are of interest.
Sir George Beilby (1921) in his monumental work on the nature of
polished metal (and other) surfaces, came to the conclusion that fine
polishing was essentially a process of flowage, in which a thin film,
500 to 1,000ppthick, was rendered mobile. This rnobile layer he consideredto behaveas a liquid and to be subject to surfacetensioneffects,
having over small time intervals, sufficient mobility to smear over and
fill imperfectionssuch as pits, scratches,and bubbles in the surfacebeing
polished.The surfacefilm "retains its mobility for an instant, and before
solidification is smoothed over by the action of surface tension" (Beilby,
1921,p. 114).On becomingrigid, this layer supposedlytook on an amorphousstructure and formed an essentially "vitreous" layer over the body
of the metal. This has sincebeenreferred to as the "Beilby layer".
N, K. Adam (1927) suggestedthat particles, possibly down to molecular size,were lifted momentarily from surfacesbeing polishedand then
redistributed, truilding up an even amorphous layer. Bowden & Hughes
(1937) following ideas put forward earlier by Macauley (L926, 1927,
1929-32) found evidence that surface temperatures at the point of
contact might, under many conditionsof sliding, be sufficiently high to
causereal melting of the metal, and that surfaceflow, polish and formation of the Beilby layer readily occurredon metals, crystals and glasses
provided that the melting point of the polishing abrasivewas higher than
that of the substancesbeing polished. They concludedthat:
(1) The process of polishing is greatly influenced biy the relative
melting points of atrrasiveand solid.
(?) Relative hardfessis comparativelyunimportant.
(3).Surface flow is brought about by intense local heating of the
surfaceirregularitiesto the melting or softeningpoint.
POLISHING. METHOD
105
( ) The molten or softenedsolid flows or is smearedover the surface,
and very quickly solidifiesto form the polished Beilby layer'
H. Raeiher (1942) in a paper of some significancegives an account of
electron and r-ray difiraction and electron microscopestudies of surfaces
polished mechanically on the one hand and electrolytically on the other'
-By
using both low angle reflection and transmission (peel) methods'
Raether-was able to study with considerable precision the behaviour
of metals when submitted to cold-working, with (amongst others) the
following conclusions:
(1) C;ld working does in fact modify the surface layers, with disruption of the original metal Ftructure.
zon.
iZl e finely recrystallised, as distinct from amorphous, surface
but
state,
produce
a
semi-liquid
not
does
is produced-i.e. cold-working
Raether,1947).
(Fig.
1,
after
fine recrystallisation
(3) Dlformation extendsto a depth of the order of-10p,with extremely
fine recrystallisation to a depth of the order of 10 A, depending on the
variables of treatment and original material.
.co *
Flc. l. Schematic representationof the upper layers of.burnished copper' showing
the very fine recrystailisation developed. From H. Raether, Mdtaux et Corrosron'
1947.
106
TIIE
CANADIAN
MINERALOGIST
Frc- 2. Taper section of abraded zinc, showing fine recrystallisation
and secondary
twinning. Taper ratio, l0:l; polarised ligirt, x aZd. ehotograpf,
ry l. d. d.-r"f".
POLISHING METEOD
107
(4) Polishing produceschangesin the properties of the surface layers
of metals. Incorporation of oxygen atoms causesan increasein resistance
to corrosion (etching) which is not developedduring electrolytic polish'
ing. More importantly, cold-hardening'is produced on a micro-scale,
thl depth of this varying in direct proportion to the duration and intensity of polishing and the soltness of the metal. Similar treatment over
fivL minutes produied a hardened layer 4p thick in copper and about
gOp thick in aluminium. Hardening apparently goes deeper than the
artificial recrystallisation.
(5) Becauseof a surplus of potential energy, the finely recrystallised
material has a tendency to coarsen,such re-aggregationbeing a function
of time and ageingtemperature.
(6) Ionic crystals differ from metals in that they are brittle, not
malleable. Hence degreeof deformation is a function of hardness.Where
deformation is produced this is by the tilting of crystal units and by the
formation of a layer of disorganised,pulverised crystal fragments, whose
size is of the order of at least 100 A diameter.
L. E. Samuels (1952-53, 1954 and in press) has carried out extensive
observations on polished surfaces of metals and alloys, and exhaustive
experimentation in the production of scratch-free surfaceswith a minimum of non-apparent deformation. He has shown (personalcommunication) that abrasion produces a finely recrystallised surface layer, which
grades down into a subsurface layer showing pronounced deformation
twins. Deepr "rays" of deformation are associatedwith individual larger
scratches. Samuels' conclusions, Which confirm those of Raether, are
particulady interesting because they have been arrived at through
orthodox optical studies----ofvery finely polishedtaper sections.Figure 2,
of a taper section of zinc prepared by Samuels, illustrates well the
development of fine recrystallisation and twinning.
Methad,sin M'i'neral,Polishing
An excellentaccount of the earlier methodsof polishing has been given
by Short (1943) and most of these are now well known. Probably the
greatest single step forward in ore-polishing technique has been the
introduction of the "Gfaton-Vanderwilt" processdescribed by Vanderwilt in 1928, in which soft metal laps were substituted for the earlier
cloth. In spite of some clear advantages,however,this method has been
found unsatisfactory in many ways and over the last few yearscontinuing
efiorts have been made to other workers to produce a quicker-and most
important-a surer method. It can be said with fair confidencethat the
great retarding factor in ore studies has simply been the difficulty of
polishing.
f08
TEE cANADTAN
MrNERALocrsr
Barringer (1953-54), largery foilowing samuers' (rgb2-bs) metailographic technique, has described a method using napped
lloth and
diamond abrasives. Hallimond (rgb4) has put forward-a method
using
diamond abrasiveson a solid nylon disc. Very recently sampson (rgb6)
has suggesteda method using diamond abrasives on silk borting croth
with high speedlaps.
Dis cuss,i.onof Mineral.ographic Metkod.s
Most of the earlier polishing methods (including the Graton-vanderwilt method) were unsatisfactory because they were incapabre
of
repeated,l'y
satisfying the first requirement of a good polish-a flat highly
reflecting surface-without at least a great deal of trouble. The more
modern methods, using diamonds on cloth, nylon and metal,
are far
more reliable, though the alternative lap types all have disadvantages,
such as excessivescratching or development-of relief, depending
on-the
rnaterial used. A further disadvantage of.all methods so iar put
forward
is that they cause excessivedeformation of the mineral surfaces,
thus
all-failing in varying degreesto satisfy the secondrequirement of good
a
polish..,
..At*:tgt vanderwilt (1928),in describinghis method, carefulry considered'the aims and effects of porishing, he paid only passing
attention
to deformation, and dismissedvery quiclry the possibiriiy or tt"
deveropment of a Beilby layer on mineral surfaces. He states (lg2g,
p. 800)
"the suggestion of a polish film is of the greatest consequence
to the
geologist, for it implies that the things seen under
the microscopeon
polished surfaces of ores are not natural geological
relationships, but
rnsteadmerely the structures of a superficiaiskin produced polishing.
by
The amorphousfilm, in so far as minerals are concernedis believed
by the
writer not to exist . . .". Berek (1g3z,noted in Turner, 1g4s)
on lhe other
hand, recognisedthe efiect of polishing on mineral surfaces,
and someof
its optical implications. Tqrner et at. (L}AE) assumedwithout
question
the development of a flowed rayer during polishing and showed
that for
stibnite Berek's characteristic angle r deireasedwith progressive
polishing, stating "The indication is that a flowed amorphous
iayer builds up
thicker and thicker as the polishing proceeds".-Ramdohr
and other
European workers have been,awarethai polishing deformation
may producemisleadingeffectsand'wandke(Ig53) has shown that
lead rap porishin_g.
may have quite a pronounced efiect on chalcocite, digenite
and
covellite and the mutual relationships of these minerals.
It has been shown irrefutabry in Fart 1 of this paper that
lead lap
methods are capable of so deforming the surface oi
iyrite, one of the
hardest of the ore minerals, that th! natural doubt"lr"ir."tion
of this
METHOD
POLISHING
109
mineral has generally been obliterated in polishing, and hencehas barely
been recognised.It is thought that this deformation takes the form of
both fragmentation and recrystallisation, resulting from the combined
action of drag and frictional heating. Most Vanderwilt-type machines
require a fairly heavy loading and as, for the highest polish, lubricant is
normally reduced to a minimirm, frictional drag is considerable.Under
these conditions it is not difficult to visualise considerabletilting and
Iragmentation of the sur{acelayers. In addition, it is common knowledge
that lead laps frequently heat during polishing, and in fact some are
water-cooledbecauseof this. If sufficient frictional heat is generatedto
warm the whole body of a large lead lap it is obvious that very consider'
able temperatures must be developed over the almost infinitely thin
mineral-lap interface. It is submitted that these two overlapping factors
of drag and heating cause mechanical distortion and recrystallisation,
the two varying in intensity and proportion principally depending upon
hardness,melting point and crystallographic orientation of the material
being polished. Such re-organisationof the surface layers into a zone of
more or less randomly oriented crystal grains reducespolarisation contrast greatly and in somecasescompletely.
Although potentially preferableto the lead lap methods from the point
of view of surface deformation, the methods of Barringer and Sampson
have a limitation in the high lap speedssuggestedand the high temperatures that result. Barringer (1953-54' p.24) states that "lap speedsdo
not appear criiical, and at a speed of 500 rpm no undue heating takes
place provided the laps are well lubricated and pressureon the specimens
is not too excessive",but then notes that the higher pressuresrequired
for polishing harder minerals may lead to heating, necessitatingreduction
of lap speed.Sampson (l'956, p. a83) mentions speeds"of 800 rpm or a
little more" and states that a speedof 850 rpm and strong hand pressure
will warm a section in 10 to 15 secondstoo hot for comfortable handling
.and in such casesmust be cooled by removing from the lap and running
water over it. He then states "However, this procedure has given some
excellent results on difficult sections." Smooth surfaces,no doubt, but
the deformationin many casesmust be considerable.
An Al,ternat'iaePol'i,shingMethod
The following method, evolved by L. E. Samuels and the present
writer in 1953,is basedon the useof awax-abrasive mixture and diamondimpregnated carrier pastes on napless cloths. As far as can be judged
from micrographs published, it producessurfacesat least comparable to
those of other methods and, as indicated in Part 1, causesconsiderably
lesssurfacedeformation.
110
THE
CANADIAN
MINERALOGIST
(l) Prel,iminary Abrasion
The abrasivepapersusedin metallographyare unsuitablefor mineralographic work as their rigidly fixed particles cause excessiveplucking in
minerals with well-developedcleavages,such as galena. In some cases,
their tearing action "starts" potential cleavagesfor somedistance below
the surface of a mineral grain, making later polishing very difficult. The
standard method of using a successionof silicon carbide or similar
abrasives on glass and/or soft metal plates is still recommended,
though it must be emphasizedthat very coarseabrasives should not be
used even in the initial stages, as these seemto causedeep looseninga point consideredby Vanderwilt himself-just as do the fixed abrasives.
It has been found quite satisfactory to commencewith about 400 mesh
grade on a rotating gun-metal lap, and then American Optical Company
303$ and 305 on stationary glass plates.
(2) Interrned,iateAhrasion-cast abrashe-war l,ap
The purpo3d of this lap is to improve on the finish obtained by the
normal preliminary abrasion processby producing a very flat surface
and by repairing most of the abrasion damage. In providing a real
intermediate step, its use greatly reducesthe requirements of the subsequent polishing stages.
The lap developed for metallographic purposes (Samuels, 1952-53)
has been found to be equally successfulin mineralography, and it cannot
be emphasizedtoo strongly that it is one, if not the, key stage of the
present process. Its value in mineralography lies in the fact that its
action is intermediate between that of a fixed and that of a rolling abrasive; the bonding of the particles is sufficient to produce the high cutting
rate and the flat finish characteristic of a fixed abrasive, but it is not so
rigid as to causetearing,the disadvantagenoted with the abrasivepapers.
The limitation qf the wax lap is that it can be used for one, or at most
two, specimensbefore requiring cleaning. However, an important feature
of the particular-lap recommendedis that it can be cleaned with great
ease,simply by wiping over with cotton-wool moistened with a solvent
such as benzene.The lap is usedstationary.
The lap itself is made of a circular metal base upon which a slab of wax-abrasive
mixture is cast. The most convenient' metal is probably aluminium; a disc of about 6 in.
diameter is cast and machined, and the surface then sawn to grve two sets of grooves
at right angles, each set parallel, with I/2 in. centres and 1/16 in. wide and deep, Such
grooves are nece$saryto secure the wax to the metal, and this particular pattern prevents
undue cracking of ttre wax on solidifrcation. Wax (microcrystalline, melting point 1?0o C.
to l90o C.) is melted and decanted to remove impurities. Silicon carbide pbwder (10 to
20 p grade) is added in stages until it is impossible to incorporate any more, at which
stage. it should be possible to push tle stiff wax-abrasive mixture cleanly away from
the side of the dish with the stiring rod. During preparation of the mixture the metal
POLISIIING METHOD
111
base should be fitted with a paper dam, and pre-heated. The wax-abrasive is then
distributed evenly over the base plate and allowed to flatten and slowly cool.
Such a wax-abrasive slab, of about 1/4 in. thickness, should last for 300 to 400 sections,
and when worn thin is simply renewed as above.
(3) Poli,shing with D.i.amond,Abrasives
(a) Application of abrasive-It
has been shown previously (16) and is
now generally accepted that diamond abrasives are most efficiently and
conveniently used when dispersed in a carrier paste. Generally it has
been found quite satisfactory to prepare the paste in the laboratory
and the following mixture has been devised by Samuels:
The components of a batch of oaste of convenient size are as follows:
Stearic acid.
Triethanolamine.....
6.0m1.
Water .
25.0 ml.
Diamond abrasive
0.5 gm.
The stearic acid is melted and heated to 80o-90oC. The triethanolamine and most of
the water are mixed and heated to the same temperature range, a small amount of
wetting agent and the diamond dust are added, and the abrasive shaken into a uniform
suspension.The molten stearic acid is stirred vigorously (preferably with a mechanical
stirrer) and the abrasive suspension introduced rapidly. The water not used in the
original suspensioncan then be used to wash in any abrasive remaining in the container.
The mixture emulsifiesimmediately, but stirring should be continued until the emulsion
cools and thickens. The paste can then be stored in,.and dispensedfrom, tinJined lead
collapsible tubes.
A total of about I+ to2" of paste is applied to spots over the central
area (about 3" diameter) of the lap and lightly rubbed in with the fingertip. Cleanliness is obviously necessary here. A few drops of kerosene
should be added to lubricate.
(6) Polishing cloths-Napped
cloths are unsuitable for high-quality
mineralographic work because the development of severe relief between
individual minerals is inevitable with their use. Hallimond appreciated
this point and developed a lap using a solid nylon disc. The relief was
then well within acceptable limits, but the lap appears to be relatively
slow in action and produces numerous rather severe scratches.
Extensive trials have shown that a napless cloth-a
heavy cotton
the most suitable. Little or no relief between constituents is
drill-is
developed, the lap has a comparatively high cutting rate, and only
minor scratches are produced. It may be used for extended periods with
mineral specimens, and consequently is economical to use. An important
advantage of this type of cloth is that it shows no tendency to damage
any of a wide range of mineral constituents so far investigated. Damage
produced during preliminary abrasion may therefore be repaired with
certainty though, because of the high cutting rate of the diamond
abrasives and the relatively high quality of the finish obtained on the
abrasive-wax lap, this rarely takes more than about two minutes.
tLz
THE
CANADIAN
MINERALOGIST
(c) Gradesof abrasive-Qualitative tests have shown that under these
particular conditions of use, diamond abrasives can be used effectively
only in the 0-10p size range. For the first polishing a 4-8p grade seems
the most suitable, and should be used on a low speedmechanical wheel
(about 150 ipm). The major polishing operation is carried out here, and
only a brief finishing treatment on a hand, or low speed, lap charged
with 0-1p grade is then necessaryto produce a finish adequate for all
routine visual examinations.
Each charge of the coarsergrade should be sufficient for about thirty
averagespecimens,and the finer grade, which producesmuch lesscutting
debris, should last for rather more. One or perhaps two re-applications
may be made to the soiledcloth, which should then be washedthoroughly
in hot soapy water, dried, again fitted to the wheel and re-chargedwith
abrasive.
@) Firnl, Polishingl
For fine photomicrography, a final polish is desirable to remove the
minor scratchesproduced on the diamond abrasive pad. It is considered
that a napped cloth, such as a synthetic suede,is essential for the final
polishing stage if the desired freedom from scratches is to be achieved
consistently. The disadvantage of napless cloths and solid laps here is
that the polishing debris accumulateson the lap surfacecausing scratching. In the case of napped cloths, on the other hand, the debris can
migrate away from the polishing surface to lodge at the baseof the nap.
It is thought thatanyvisible scratchesproduced on this final pad are
caused by the polishing cloth rather than by the abrasive and on this
basis the following final polishing technique has been evolved. Calcined
magnesiumoxide (from which air has been carefully excluded) is mixed
into a very thick paste with water and a drop or two of detergent and
pushedthrough a fine sieveonto the polishing pad, such sieving treatment
serving to break up the paste into a thick cream.This paste is then spread
over the surface of the polishing cloth and the specimenmoved lightly
over it, so that the surface being polished skids over a packed bed of
paste without touching the nap of the cloth. The lap itself is stationary.
This technique is capableof producing surfaceswhich, even in the softest
low-melting-point metals, appear scratch-free under phase contrast
illumination.
Such treatment does produce some relief between constituents. The
lThroughout the process, from coarsest abrasion to final polishing, hand pressure
should be decreased towards the end of each stage such that the final movements
approach a light wiping action. This reduces the effective particle size of the abrasive,
so producing a more even gradation through the polishing process. This. is important.
POLISHING METIIOD
113
minor relief resulting from relatively short treatments is, however,
frequently an advantage,as it increasesthe contrast betweenthe different
minerals.There is a limit to this, of course,and the treatment shouldbe
terminated before relief becomesserious. In view of this limit to the
finishing treatment, it is of some importance to ensure that the finest
possiblefinish is obtained on the last diamond abrasive pad.
Il,lustration of Results
The accompanying photomicrographs have been chosen to give an
indication of the quality of the averagepolish obtained with routine use
of the method described. The minerals illustrated range in Talmage
hardness (Uytenbogaardt, lgbl) from F- (pyrite) to B+ (covellite).
Where possible,hard and soft minerals (e.g. pyrite and galena in fig. 4)
have been shown in contact so as to give an indication of maximum relief
likely and a comparison of the quality of polishesobtained. Attention is
drawn to the diffuse chalcocite-covellite reaction rim about galena in
figs. 10 and lL which, although readily observedin these illustrations is
barely discernible after polishing by metal lap methods.
A feature of all the illustrations is the absenceof major scratches.
In one or two instances(e.g.figs. 9 and 1l), fine scratchesare discernable;
had a little more time been spent on these particular surfaces, such
scratchescould have been eliminated. However, since the purposeof this
paper has been to describe a method of routine application, it was
thought best to submit photographs of polishesobtained in the normal
run of work, rather than of those producedby unusually careful preparation. The averagetime required for preparation of the surfacesillustrated
was about six to sevenminutes, with a maximum of about ten. The sp,eed
with which such polishes can be obtained is clearly a most important
practical advantageof the method.
Somelrnpl,ications of the Method.
A method which permits the examination of material more closely
similar to the natural substancesbeing investigated clearly opensa field
for the more accuratedetermination of hardness,reflectivity, polarisation
and other properties which depend on the crystal structure. The more
refined methodswhich have recently beendevelopedto measurehardness
and reflectivity are very possibly already measuringdegreesof distortion
and giving readingsvarying accordingly; in fact it is suspectedthat the
accuracies of some of these instruments may be so much vrithin the
Iimits of variation-caused by deformation-of the respectiveproperties
that variations in readings are being given unmerited significance.The
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POLISHING
METHOD
rt7
work of Turner et al'. (L945) showsclearly how surface deformation may
effect, quantitatively, the polarisation effects of some minerals.
Preliminary observationsnow being carried out on somecubic minerals
(principally cobalt and nickel sulphides and related minerals) suggests
that a number of these, in addition to pyrite, may be generally doubly
refracting. Anomalous anisotropism has of course been recognised in
minerals such as cobaltite, gersdorffite, argentiferous galena and so on,
but it seems likely that such observations will become much more
frequent with the use of a finer polishing method. Apparently the film
of distorted material has been just sufficient to mask the feeble double
refraction produced by cubic minerals which possessa low symmetry
or whose structures, owing to variations in composition, are slightly
distorted. A further point of interest is that minerals such as chalcocite,
already known to causeweak double refraction, show this with greater
intensity and sharpness urhen polished with a minimum of surface
deformation.
Acknowled,gments
It is the writer's pleasureto acknowledgethe generoushelp of L' E.
Samuels,whoseresearchesin metallographic polishing have provided the
basis for much of the present paper, and who has given unselfishly of his
time in discussion and experiment. Thanks are also due to Professor
J. E. Hawley and Dr. L. G. Berry of Queen'sUniversity, and to Dr. C. G.
Suffel of the University of Western Ontario for their careful reading of
the manuscript.
RnpsnsNcBs
Aoeu, N. K. (1927): Nature, 119,279.
BAnnwcen, A. R. (1953-54): The preparation of polished sections of ores and mill
products using diamond abrasives. Bul'L. Inst, Min. Met. 63,21.
Brner, M. (1937): Optische Messmethoden im polarisierten Auflicht, Fort. Min. Petr.
22.
Benry, Srn GBonoo (1921): Aggregotinnof flotu of sol'id's.London, Macmillan and Co.
BoworN, F. P. & Hucurs, T. P. (1937): Polishing, surface flow and the formation of the
Beilby layer. Proc. Roy, Soc. (A) 160, 575.
CeuenoN, E. N, & Gnrsr, L. H. (1950): Polarisation figures and rotation properties in
reflected light and their application to the identification of ore minerals. Econ.
Geol'.45,7L9.
Gnsex, L. H. (1952): The relationship between polarisation colours and rotation
properties of anisotropic minerals. fuon, Geol.4il, 45L.
HArrtMottD, A. F. (1964): Polishing mineral specimens. The Mining Mag 9l (4),
208.
MAcAuLAy, J. M. (1926): Nature, ll8, 339.
(1927): Nature, 119, 113.
(1929-1932): J.R. Tuh. Col,l'.Glasgow, 2, 378.
118
THE
CANADIAN
MINERALOGIST
Reelurn, H. (1947): La structure des surface polies mechaniquement et electrolytiquement. Metaax et Corros,i,on,22,2.
Saurson, E. (1929): The determination of anisotropism in metallic minerals. fuon.
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(1949): A method for polishing sections of ores. fuon. Geol. U, LL9,
(1956): A procedure for polishing ore sections with diamond abrasive. Econ.
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Seuuer.s, L. E. (1952-53): The use of diamond abrasives for a universal system of
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(1954): The practical applications of a system of metallographic polishing
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Snort, M. N. (1948): Microscopic determination of the ore minerals. U.S. Geol,.Sunse!,
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TunNun, A. F., Bnrvronn, J. R. & Mcl,eer, W. J. (1945): A polarised light compensator
for opaque minerals. Econ. Geol,.40, L8,
UyrsrcnoGAARDr, W..(1951): Tabl,esfor whroscopic itenti,fratian of ore mineral,s.
Princeton Universitv Press.
VArorr.wrlr, J, W. (19iS): Improvements in the polishing of ores, fuon. Geol,.2Z,?f/2.
Waxnrr, A. D. (1953): Polishing phenomena in the copper sulphides. fuon. Geol,.
48,225.